Charge confinements in CdSe–ZnSe symmetric double quantum wells

Abstract

The bound states in the (CdSe)Nw(ZnSe)Nb(CdSe)Nw–ZnSe(001) symmetric double quantum wells are investigated versus the well width(Nw) and the barrierthickness (Nb). Acalculation based on the sp3s* tight-binding method which includes the spin–orbit interactions is employedto calculate the bandgap energy, quantum-confinement energy, and bandstructures. The studied systems possess a vanishing valence-band offset(VBO = 0) in consistency with the well known common-anion rule, and a large conduction-bandoffset ( eV), which plays an essential role in the confinement of electrons withinthe CdSe wells. The biaxial strain, on the other hand, plays another rolein confining the holes at the interfaces (within the well regions) and thusenhancing the radiative efficiency. The induced-strain energy is estimated to be∼35 meV. More importantly, the results show that, for a fixed barrier thickness, the double wells areable to confine a pair of bound states when they are very thin. By increasing the wells’ width(Nw), further, a new pair of states from the conduction-band continuum falls into the wells every timeNw hits a multiple of four monolayers (more specifically, for4n<Nw≤4(n+1), the number ofbound states is 2(n+1), where n is an integer). On the other hand, the barrier thickness(Nb) is shown to have no effect on the number of bound states, but it solely controlstheir well-to-well interactions. A critical barrier thickness to switch off these latterinteractions is estimated to occur at about (). Rules governing the variation of the quantum-confinement energy versus both barrier thickness(Nb) and wellwidth (Nw) have been derived. Our theoretical results are also shown to have excellent agreementwith the available experimental photoluminescence data.